Recent concern about the environment has accelerated efforts by industry and government throughout the world to seek solutions for pollution problems applicable to air and water. The present invention relates to a process for improving the quality of air in and near coke plants by controlling the emissions which normally arise when coke is pushed from a coke oven.
Coke is produced by heating coal in the absence of air in by-product ovens that are designed and operated to collect volatile products given off by the coal during the coking process. The ovens are built of refractory brick, are long and narrow and tapered from one end to the other, i.e. narrow at one end and slightly wider at the other end, so that coke can be easily pushed from the ovens. The ends of each oven are equipped with removable refractory-lined doors that are positioned in place and sealed, either by self-sealing devices or a sealant, during the coking process. Coal is charged to an oven through several roof openings called charging holes that are closed by cast iron lids after the oven is charged. A plurality of ovens built in side-by-side relationship form a structure called a battery, which may comprise as many as eighty or more ovens. The side of a battery at the narrow ends of the ovens is called the "pusher-side" and the opposite side of the battery at the wider ends of the ovens is called the "coke side". Each side of a battery is equipped with a bench, which serves as a working platform, at an elevation above ground level but below that of the bottoms of the ovens. One or more batteries is referred to as a coke plant, and associated with such a plant is one or more coke quenching stations, the locations of which depend upon the relative positions of the batteries.
The operation of a coke oven battery is supported by several large mobile machines. Even oven is designed to take a definite volume of coal that is charged from a larry car which operates on rails that extend longitudinally of the top of the battery. At ground level and extending longitudinally of the pusher side of the battery, but independent from it, is a track upon which a pusher machine travels. The pusher machine is usually a combination door extractor, leveler and pusher. The door extractor removes a pusher side door from sealing engagement with an oven, holds the door while the oven is pushed and then repositions the door on the oven. The leveler is run through an oven a short distance from its top shortly after the oven is charged with coal to level the coal. The pusher includes a large reciprocating beam with a ram or enlarged head and is extended through the oven from the narrow pusher side end through the wider coke side end to force the coke from the oven at the end of each coking cycle. Extending longitudinally of the coke side of the battery and on the coke side bench is a door extractor machine, which operates in a similar fashion to the one on the pusher side and which positions a coke guide that is coupled to the door machine. The coke guide is aligned with an oven from which coke is pushed and provides a passage for the hot coke, across the coke side bench, before it drops into the quenching car. At ground level and extending longitudinally of the coke side of the battery, but independent from it, is a track upon which a quenching car travels. The track extends to a quenching station where hot coke in the quenching car is quenched with large volumes of water.
For many years the typical coke oven built in the United States had a coking volume of about 500 cubic feet, a coking capacity of about 9 tons and was about 37 feet long, 10 feet high, and had an average width of about 18 inches. The normal coking time or coking cycle for coke in such an oven was about 18 hours or a coking rate of about an inch an hour. The quenching car serving such a battery had an open top, was slowly moved during the discharge of coke from an oven into the car, was designed to catch a relatively uniform bed of coke about two feet thick on the sloping bottom of the car, and was about 40 feet long. The car was equipped with power operated gates on the side of the car farthest from the battery and at the lower side of the sloping bottom. The gates were closed during the discharge of coke into the car and while it moved to and from the quenching station and were opened to discharge coke from the car. Holes or slots in the gates permitted water not evaporated during the coke quenching process to drain from the car during and after quenching.
Most modern coke oven batteries built in the world during the past fifteen years have ovens substantially larger than those built earlier to take advantage of labor savings associated with the operation of large batteries. A large oven may have a coke volume of about 1400 cubic feet and a coke capacity of about 23 tons or more. The ovens may have a length of about 50 feet, a height of 20 feet or more and an average width of about 18 inches. Like the older batteries, the modern ones have a normal coking time or coking cycle of about 18 hours, i.e. a coking rate of about an inch an hour. The quenching car serving a battery of large ovens was initially a larger version, possibly 65 to 75 feet long, of the older moving car. However, during the past few years there has been a trend toward a quenching car that is designed and operated in a different manner than the moving car. The new quenching car is designed to receive the entire volume of coke discharged from an oven while the car remains stationary and is referred to as a single-position or one-spot car. A one-spot car may have a length of about 20-25 feet, substantially shorter than the moving coke cars of earlier design.
Government agencies have established regulations that define acceptable limits for pollutants discharged into the air, and industry is seeking ways to comply with such regulations. One such regulation states: "No person shall operate any equipment so as to produce, cause, suffer, or allow smoke or other visible emissions in excess of 40 percent opacity for more than a cumulative total of 15 minutes in any 24 hour period." An accepted visual method for determining opacity for air pollutants is by the RINGELMANN system, which is described in "Air Pollution Handbook", edited by Magill, Holden and Ackley, McGraw-Hill Book Co., Inc., 1956, with particular reference to section 6.7, pages 6-33 to 6-36, which section is hereby incorporated by reference. Opacity rating is defined in "Industrial Pollution Control Handbook", edited by Herbert F. Lund, McGraw-Hill, Inc., 1971, Glossary, p. 21, as "(A) measurement of the opacity of emissions, defined as the apparent obscuration of an observer's vision to a degree equal to the apparent obscuration of smoke of a given rating on the Ringelmann chart." Smoke emission standards limit emissions in terms of smoke density on the Ringelmann scale, the scale numbers varying on a scale from 0 to 5 and corresponding to a "percent blackness" of 0 to 100.
In order to comply with such regulations industry has directed substantial efforts toward the reduction of emissions from coke plants. Those emissions may be categorized as intermittent or continuous in nature, and this invention is directed to the reduction of intermittent emissions, specifically those associated with coke oven pushing operations. The pushing operation may be defined as the operation by which hot coke is removed from a coke oven and transported to a quenching station, beginning when the coke side door is first removed from a coke oven and continuing until the quenching of hot coke is commenced. Pushing emissions may be defined as any air contaminant, particularly visible smoke and fine solid particles of coke or incompletely coked coal, which is generated by or results from the pushing operation.
Emissions vary widely from battery to battery and relate to the battery age, design, maintenance and operation, particularly charging and heating practices and the allotted coking time cycle. Emissions are caused by a variety of factors. Coke pushed from any oven crumbles as the coke falls from the guide into the quenching car producing grit or fine particles in varying amounts. Coke from the corners of an oven includes some incompletely coked coal and tar that burns and releases smoke as the coke is pushed from the oven. Extreme temperature difference between hot coke and the outside air produces strong air currents that carry particles of coke and uncoked coal into the air and accelerate the burning of combustible solids and gases. Finally, as a quenching car of hot coke travels from the oven from which the coke was discharged to the quenching station, the passage of air through the coke causes air currents that carry off fine particles of grit and continue combustion of the hot coke.
No reference to coke oven pushing emissions would be complete without a discussion of coke quality. The length of coking time determines to a large degree the "quality of coking" which is defined as the "greenness" of the coke pushed from an oven. Green coke is coke from which the volatile matter has not been completely distilled by the coking process and is evidenced by heavy black emissions rising from a coke quenching car that is receiving hot coke pushed from an oven. An experienced observer is able to make a judgement as to the quality of coke pushed from an oven based on the degree of "greenness" as e.g. very green, green or moderately green as compared to a "clean" push, i.e. one giving off little or no visible emissions. An important factor to be considered with respect to green pushes is that they can generate combustibles such as CO and H.sub.2 in explosive concentrations when confined and contacted with air. For example 6 mol percent of CO and H.sub.2 in equal parts or 4% H.sub.2 are explosive concentrations. Any system for controlling coke oven pushing emissions must be designed to control emissions created during the discharge of green coke from an oven.
Numerous systems and devices have been suggested for controlling coke oven pushing emissions. Reference is made to two papers, "Coke-Oven Air Emission Abatement", Iron and Steel Engineer, October 1972, pp. 86-94 and "Control of Coke-Oven Emissions", Ironmaking and Steelmaking, 1975 Volume 2 Number 3, pp. 157-187, which describe several such systems including the following.
(1) Bench-mounted self-contained hood system includes a coke guide associated with a hood that is connected through ducts to a gas scrubber. The gas scrubber is either mounted on the coke guide or a car coupled to the guide, and the hood is designed to extend over an open-top moving quenching car, for its full width and a portion of its length, to collect emissions that arise when hot coke is discharged into the quenching car.
(2) Hot car mounted enclosure and hood system, referred to as a "mobile" system, includes a one-spot quenching car upon which is mounted a hood that is designed to fully enclose the top of the quenching car but has an opening that cooperates with the end of a coke guide. The opening permits coke pushed from the guide to fall into the car when coke is discharged from an oven. Emissions from the coke are withdrawn from the quenching car through ducts connected to a scrubber system that is mounted either on the car or a coupled trailer.
(3) Fixed duct hood and scrubber system, referred to as a "land-based" system, includes a separate emission collecting main that extends along the coke side of a battery and has a valved outlet adjacent each oven and a fan and scrubber associated with the main. A hood and ductwork are associated with the coke guide and connect with the collecting main during the time when coke is being pushed from an oven into a moving open-top quenching car which covers the width of the car but only a portion of its length.
While each of the above systems has been somewhat successful in reducing emissions, the following disadvantages are known:
(1) The bench-mounted self contained hood system is large and heavy and extends over a significant portion of the coke side bench. Furthermore, the efficiency of emission collection is greatly affected by the large gap between the bottom of the hood and the moving open-top sloping-bottom quenching car and there is no demonstrated way to control emissions from the hot coke from a normal coking cycle when the car moves from under the hood and travels to the quenching station.
(2) The mobile system car may have a length of 100 feet or longer. The system will be difficult to apply to existing coke plants where space is limited, the capital and operating costs are high and a special quenching system is required in conjunction with the enclosed-top one-spot quenching car. Mobile systems are just appearing in the United States and the special problems associated with the excessive car length, weight, and special quenching systems remain to be solved on a production basis.
(3) The land-based system emission collection efficiency is greatly affected by the large gap between the bottom of the hood and the moving open-top sloping-bottom quenching car and there is no demonstrated way to control emissions from the hot coke from a normal coking cycle when the car moves from under the hood and travels to the quenching station. In Japan where the land-based system is used extensively, the normal coking cycle is extended for a "soaking period" during which the coke is further heated in the oven to assure that all the volatile matter has been driven off and the coal is completely coked. The soaking period minimizes any pollutants being given off while the hot coke in a car moves to the quenching station, but there is a considerable production penalty associated with the soaking period. In plants where the bench mounted self-contained hood system and the land-based system have been tried the drafting and exhausting equipment has had to be enlarged, resulting in even higher operating costs, in an attempt to improve emission collecting efficiency.